- Objectives
- Overview
- Coordinates for 3D Cad Modeling
- Geometric Entities
- 4.1 Manually Bisecting a Line or Circular Arc
- 4.2 Drawing Tangents to Two Circles
- 4.3 Drawing an Arc Tangent to a Line or Arc and through a Point
- 4.4 Bisecting an Angle
- 4.5 Drawing a Line through a Point and Parallel to a Line
- 4.6 Drawing a Triangle with Sides Given
- 4.7 Drawing a Right Triangle with Hypotenuse and One Side Given
- 4.8 Laying Out an Angle
- 4.9 Drawing an Equilateral Triangle
- 4.10 Polygons
- 4.11 Drawing a Regular Pentagon
- 4.12 Drawing a Hexagon
- 4.13 Ellipses
- 4.14 Spline Curves
- 4.15 Geometric Relationships
- 4.16 Solid Primitives
- 4.17 Recognizing Symmetry
- 4.18 Extruded Forms
- 4.19 Revolved Forms
- 4.20 Irregular Surfaces
- 4.21 User Coordinate Systems
- 4.22 Transformations
- Key Words
- Chapter Summary
- Skills Summary
- Review Questions
- Chapter Exercises
This chapter is from the book
This chapter is from the book
This chapter is from the book
Overview
Engineering drawings combine basic geometric shapes and relationships to define complex objects. 2D drawings are composed of simple entities such as points, lines, arcs, and circles, as well as more complex entities such as ellipses and curves. Reviewing the basic geometry of these elements helps you define and combine these elements in your drawings and CAD models.
Accurate construction is critical to creating useful drawings. Lines drawn using a CAD system are highly accurate definitions—much greater than you can see on a computer monitor. Good manual drawing technique can typically produce a drawing accurate to about 1/40th of the drawing scale. For example, a hand-drawn survey created at 1″ = 400′ might be accurate to a range of plus or minus 10′. The internal precision of drawings created using CAD systems is limited by the 64 bits (base-2 places) typically used to represent decimal numbers in a CAD system. This produces a theoretical accuracy of around 1 in 10 quadrillion (1016). If you drew two beams, each three times the distance from the Sun to Pluto, and made one of the beams just 1 mm longer than the other one, a CAD system could still accurately represent the difference between the two beams. Wow! That’s a lot better than the 1 in 40 accuracy of a manual drawing. However, CAD drawings are accurate only if the drawing geometry is defined accurately when the drawing is created.